For operations involving patients who require multiple hip surgeries, it is often necessary to replace a femoral hip implant with a long stem hip implant. That is, a short stem hip implant is not generally used for revisions. Long stem hip implants can also be used in other situations. For example, they can be used in patients undergoing revision surgery or requiring an implant with a long stem because of a fracture in the femur or a bone defect (for example as a result of tumor removal).
However, when such a long stem hip implant is fully implanted, it is very important that its distal end be centered in the medullary canal. If the distal end is not centered, a non-uniform cement mantle will result as the implant is inserted into the canal containing cement. Then, unequal stresses on the cement mantle may result in failure of the cement mantle. Such failure can eventually result in loosening of the implant.
Additionally, because long stem hip prostheses generally extend in the femur beyond the region referred to as the isthumus of the femur (where the cortical bone thickens and the opening in the intramedullary canal decreases), correct placement of a long stem prosthesis presents a special challenge to the physician.
Broaches are often used to prepare the medullary canal. These broaches can be designed to rotate slightly into anteversion (i.e., the neck of the broach rotates forward as the broach is driven into the bone). The final orientation mimics the natural anatomic orientation of the femoral neck. This rotation (which is approximately 10.degree.) presents no problem in the alignment of a short prosthesis, wherein the stem is not long enough to impinge on the curved section of the femur, particularly in the region of the isthmus. However, when a long, bowed stem is used as a prosthesis, the rotation will often result in poor placement of the distal tip of the implant.
It is known in the prior art to have "anatomically" shaped prostheses which have been made with simple distal bows and/or distal and proximal curvatures. However, the distal tip of the implant must still be correctly positioned, as described above. The problem of positioning an implant within the isthmus of the bone has not previously been adequately addressed in the prior art.
Additionally, for economic purposes, it is desirable to have a limited number of ready-made long stem hip implants for use by the population requiring such implants.